In a two-way satellite communication system, comprised of a hub and plurality of remote terminals (e.g., very small aperture terminals (VSATs)), information from VSATs may be transmitted towards the hub (and in case of mesh topology between VSATs as well) over shared bandwidth resource, known as return channel. The return channel may further comprise one or more transmission channels at one or more symbol rates (i.e. transmission speeds). Each transmission channel may occupy at least a portion of the return channel bandwidth without overlapping any of the other transmission channels. Furthermore, each transmission channel may be further divided in the time domain to one or more timeslots, wherein each timeslot may be used for transmitting a single data burst. In addition, in some embodiments of such satellite communication system, said timeslots may be configured for transmission of data bursts using one or more modulation and (forward error correction) coding combinations.
In such satellite communication system, use of the return channel may be governed by an access scheme, i.e. a method according to which each VSAT may determine, either on its own or through receiving allocation information from the hub, one or more timeslots over one or more transmission channels for transmitting information. Such access scheme, which may allow multiplexing of VSAT transmissions over both frequency and time, is often referred to as Multiple Frequency Time Division Multiple Access (MF-TDMA).
In many embodiments of MF-TDMA access schemes, a predefined time-frequency map may be known both to the hub and to the VSATs. In some embodiments, such as Digital Video Broadcasting-Return Channel via Satellite (DVB-RCS) (EN 301 790), the hub may distribute the predefined time-frequency map to VSATs over the forward channel (which may be a statistically multiplexed channel for transmitting information from the hub to the VSATs). In other embodiments, the time-frequency map may be calculated by a hub and by VSATs separately based on configuration parameters (which may be programmed into the hub and the VSATs during installation or thereafter).
Since time is endless but computation and memory resources are finite, the time-frequency map may describe the return channel composition (e.g. in terms of transmission channels and timeslots) for a limited period of time, whereas the same return channel composition pattern may be endlessly repeated. In addition, a VSAT may select one or more timeslots and/or the hub may allocate one or more timeslots to a VSAT (for at least the purpose of transmitting one or more data bursts) with reference to a time-frequency map, i.e. on one or more predefined transmission channels and at one or more predefined timeslots.
In some embodiments, the MF-TDMA access scheme is combined with a reservation technique, i.e. a VSAT may use only those timeslots which may be allocated to it by the hub. In order to receive such allocation, a VSAT may send a capacity request to the hub over a signaling channel (e.g. dedicated timeslots, which a VSAT may be allocated at fixed intervals, or shared timeslots which one or more VSATs may utilize in a contention manner, for at least this purpose). In such embodiments, a method at the hub for allocating timeslots to a VSAT may include a step of considering the transmission capabilities of the VSAT (e.g. in terms of transmission power) and its current link conditions (e.g. fade conditions). Such step may be necessary for allocating timeslots of appropriate symbol rate, modulation and coding characteristics, at least for the purpose of ensuring at an applicable destination, be it the hub or another VSAT in case of mesh connectivity, appropriate reception conditions for a burst transmitted by the VSAT.
However, due to time-frequency map limitations, such as map distribution intervals (which are often quite long in order to minimize bandwidth usage for this purpose over the forward channel) and predetermined timeslot characteristics, existing allocation methods lack dynamic versatility (either the time-frequency map never changes, or changes are rare due to the relatively long intervals required for gathering sufficient statistics regarding return channel usage and adjusting the time-frequency map accordingly). Such methods do not allow rapid changes and/or adaptation in real time (i.e. at the time of allocating bandwidth to VSATs) to changing satellite link conditions, either at the hub side or at the VSATs side. This often results in far from optimal use of space segment resources and consequently in either increase of network operational costs (due to needing more space segment) or in degraded service.